1. Technical Field
The present invention relates to a polyoxymethylene composition which is improved in heat stability, particularly little generating formaldehydic stench in molding, and in the occurrence of mold deposits.
More particularly, the present invention relates to a polyoxymethylene composition which exhibits excellent antistatic properties and is improved in the occurrence of mold deposits, one which is excellent in the resistances to weather and heat and is improved in the occurrence of mold deposits, one which exhibits excellent impact resistance and is improved in the occurrence of mold deposits, and an electrically conductive polyoxymethylene composition which is improved in the occurrence of mold deposits.
2. Description of the Related Art
A polyoxymethylene resin is excellent in mechanical properties, resistances to fatigue, wear, abrasion, chemicals and heat, and moldability, and has therefore been utilized in various fields including automobile, precision machines such as electrical and electronic appliances, building materials, piping and so forth. With enlargement and diversification in the application field, however, there has arisen a tendency to require the resin to have higher-level quality.
As one of properties to be demanded, there is cited that the reduction of the mechanical strengths thereof in its extrusion or molding step, the occurrence of deposits formed on the mold (i.e., mold deposits) or the like should be kept under controll in a lowered level. One of the chief factors causing such unfavorable phenomena is the decomposition of the polymer by heating.
Polyoxymethylene has an essential property of being easily decomposed in a hot oxidizing atmosphere or under an acidic or alkaline condition owing to its chemical structure. With respect to a polyoxymethylene homopolymer, a method wherein the chemically active ends thereof is esterified (e.g., acetylation), or a method wherein trioxane is copolymerized with a monomer having an adjacent carbon-carbon bond (such as a cyclic ether or a cyclic formal) and then the unstable ends of the obtained copolymer are decomposed, for example, are known for the stabilization of the chemically active ends of the polyoxymethylene homopolymer. However, a polyoxymethylene resin also suffers from the cleavage of the backbone chain in heating, and this phenomenon cannot be inhibited by the above means alone, thus necessitating the addition of an antioxidant and other stabilizer in practice.
The antioxidant to be added to a polyoxymethylene resin includes sterically hindered phenol compounds and sterically hindered amine compounds, while the stabilizer other than antioxidants includes polyamides, urea derivatives, amidine compounds, hydroxides of alkali or alkaline earth metals and salts of these metals with organic or inorganic acids. Various combinations of these antioxidants with these stabilizers are used.
However, even a polyoxymethylene composition containing these stabilizers is not sufficiently prevented from suffering decomposition of the polymer. Actually, when such a composition is exposed to heat and oxygen in a molding cylinder during molding, the composition generates formaldehyde by the cleavage of the backbone chain or from insufficiently stabilized ends to worsen the working atmosphere in extrusion molding. Further, when the molding work is prolonged, powdery or tarry deposits (mold deposits) adheres on the mold surface, which is one of the chief factors causing lowering in the working efficiency and the deterioration of the surface appearance of a molded article. Further, the decomposition of the polymer brings about lowering in the mechanical strengths thereof. Although various attempts have been made under these circumstances to find a more effective stabilizer formula, no satisfactory result has been obtained as yet. For example, a proposal to add a melamine-formaldehyde polycondensate together with an antioxidant and other heat stabilizer to polyoxymethylene is disclosed in Japanese Patent Publication-A No. 33943/1977. This attempt is effective at inhibiting troubles resulting from the sublimation of melamine, i.e., the blooming of melamine from a molded article or the adhesion of melamine to the mold, because of the use of a highly crosslinked melamine-formaldehyde polycondensate. However, this attempt sacrifices the formaldehyde-trapping power of melamine to obtain the above effect, so that the addition of the above components is inferior to that of melamine in the effect of inhibiting the formation of mold deposits resulting from the generation of formaldehyde gas.
Further, a polyoxymethylene resin as well as many other plastics has a high specific surface resistance, so that it suffers from troubles due to static charge buildup, for example, electrostatic noise, surface staining and/or adhesion of dust, when used in some application fields.
Various antistatic agents are on the market to inhibit these troubles and they are useful also for polyoxymethylene. However, for example, ionic substances are causative of discoloration of polyoxymethylene and lowering in the heat stability thereof unfavorably. Although nonionic antistatic agents are known as an antistatic agent which is free from such adverse effects, the nonionic antistatic agents are generally poor in antistat effect. Therefore, a large amount thereof must be added to lower the electrostatic chargeability to a level of practical use. Meanwhile, such an antistatic agent generally acts also as a lubricant, so that the addition thereof in a large amount lowers the mechanical characteristics of a molded article and the processability in molding. Further, the use of, especially, a solid antistatic agent such as glycerol monostearate gives waxy bloom on the surface of a molded article, while the use of a liquid one liquid bloom thereon. Thus, the addition of such an antistatic agent deteriorates the molded article in appearance and function, and particularly the addition thereof in a large amount further brings about lowering in the heat stability.
As compounds which can lower only the electrostatic chargeability of polyoxymethylene sufficiently without causing the above troubles such as discoloration, excess blooming, lowering in the heat stability, change in the moldability and so forth, which are caused by the use of the antistatic agents of the prior art, additives which have been disclosed in Japanese Patent Publication-A No. 5934/1980 which has previously been filed by the present inventors, i.e., a fatty acid ester of a polyhydric alcohol and a polyethylene glycol are known. Further, there was also proposed the addition of a small amount of an organometallic salt in order to improve the heat stability (see Japanese Patent Publication-A No. 96157/1984).
As results of these various studies, good results were achieved by these studies with respect to antistatic effect. However, no sufficiently satisfactory results could be achieved with respect to the effect of improving the heat stability of a polyoxymethylene resin containing an antistatic agent, particularly with respect to the effect of inhibiting the formation of mold deposits in the molding thereof.
Polyoxymethylene generally has an essential property of being easily decomposed in a hot oxidizing atmosphere or under an acidic or alkaline condition owing to its chemical structure, so that the addition of an antioxidant and other stabilizer thereto is indispensable in practice.
The antioxidant to be added to a polyoxymethylene resin includes sterically hindered phenol compounds and sterically hindered amine compounds, while the other stabilizer to be added thereto includes compounds such as polyamides, urea derivatives, amidine compounds, hydroxides of alkali and alkaline earth metals and salts of these metals with organic and inorganic acids. Various combinations of these antioxidants with these other stabilizers are used.
However, a polyoxymethylene composition containing these stabilizers is not sufficiently prevented from suffering decomposition. Actually, when the molding work of the composition is prolonged, the composition generates formaldehyde by the action of heat and oxygen in a molding cylinder to worsen the working atmosphere and to adhere powdery or tarry deposits (mold deposits) on the mold surface, which is one of the chief factors causing lowering in the working efficiency and the deterioration of the surface appearance of a molded article. Further, the decomposition of the polymer brings about lowering in the mechanical strengths thereof. Although various proposals and attempts have been made under these circumstances to find a more effective stabilizer formula, no sufficiently satisfactory formula has been found as yet. For example, the addition of a melamine-formaldehyde polycondensate together with an antioxidant and other heat stabilizer to polyoxymethylene is proposed in Japanese Patent Publication-A No. 33943/1977. However, the addition of these stabilizers is still insufficient for a polyoxymethylene containing an antistatic agent, so that further improvement has been expected.
Further, a polyoxymethylene resin has also been required to have unique characteristics as the material with the enlargement and diversification of the application field thereof. One of the unique characteristics is further improved weathering (light) resistance. That is, when interior or exterior automotive trim parts or components of electrical appliances are exposed to sunlight, fluorescent lamp, rain, open air or the like for a long period, favorably they discolor, lose their surface smoothness to exhibit poor gloss, or suffer from surface cracking to result in poor surface appearance depending upon the conditions used. Up to this time, an ultraviolet absorber such as a benzophenone or benzotriazole, a combination of such an ultraviolet absorber with a hindered amine light stabilizer, or a combination of such an improver for weathering resistance with a colorant such as a dye or pigment has been used in order to overcome the above disadvantages.
However, the addition of an ultraviolet absorber or a hindered amine light stabilizer in a large amount in order to attain a sufficient weathering resistance brings about lowering in the heat stability and/or moldability of polyoxymethylene and impairs the characteristics inherent in polyoxymethylene. Further, the addition of a colorant such as carbon black has a disadvantage of lowering the heat stability remarkably.
As a mean to improve the heat stability of polyoxymethylene in molding, a method wherein a sterically hindered phenol compound is added to polyoxymethylene together with a polyamide, an amidine compound, a hydroxide of an alakali or alkaline earth metal or the like, is known.
Although the use of these means somewhat improves the heat stability, the improvement is still insufficient. Particularly, when the molding work is conducted in a long-term period of time, there occur various troubles that formaldehyde is generated to worsen the working atmosphere and that powdery or tarry substances (mold deposits) adhere to the mold surface to being about lowering in the working efficiency and deterioration of the surface appearance of a molded article. Accordingly, an improvement in these respects has been expected.
Further, an example of the properties to be required in a polyoxymethylene resin includes an impact resistance. More specifically, a molded article which is used as e.g., an automobile part is desired to exhibit a high impact resistance over a wide temperature range. The addition of a thermoplastic polyurethane has been proposed as a mean for improving the impact resistance of polyoxymethylene in the prior art (see, e.g., Japanese Patent Publication-A Nos. 145243/1984 and 19652/1986).
Owing to poor interfacial adhesion between polyoxymethylene and thermoplastic polyurethane, however, these means have disadvantages that exfoliation between them occurs on the surface of a molded article to impair the surface appearance of the article and that the products of decomposition of polyoxymethylene or polyurethane, and/or exfoliate is built up on the mold surface during molding to lower the dimensional accuracy and the working efficiency of molding, though these means which have been proposed heretofore can give a polyoxymethylene composition having an acceptable satisfactory impact resistance.
As compounds which is used to improve the impact resistance and simultaneously to reduce the amount of mold deposits resulting from the products of decomposition of the thermoplastic polyurethane by improving the interfacial adhesion between a polyoxymethylene and a thermoplastic polyurethane in such compositions containing the thermoplastic polyurethane in the prior art, additives which have been disclosed in Japanese Patent Publication-A No. 132117/1990 which has previously been filed by the present inventors, i.e., isocyanate compounds, are known. Recently, however, further improvement with respect to the mold deposits formed in molding has been required.
Although the addition of a sterically hindered phenol cmpound together with a polyamide, an amidine compound, a hydroxide of an alkali or alkaline earth metal or the like has been known as means for improving the heat stability of polyoxymethylene in molding, it is still insufficient for improving the heat stability of a polyoxymethylene containing a thermoplastic polyurethane and further improvement has been required.
Meanwhile, it has been a practice to add an electrically conductive carbon black to polyoxymethylene for the purpose of imparting electrical conductivity thereto. However, the addition thereof in an amount enough to impart the desired performance to polyoxymethylene causes a problem of lowering the heat stability of polyoxymethylene remarkably. In order to enhance the heat stability, the addition of a specific amide compound which is disclosed in Japanese Patent Publication-A No. 51937/1984 (Japanese Patent Publication-B No. 31736/1986) which has previously been filed by the present inventors, is known. However, the addition thereof could not always give satisfactory results with respect to the effect of inhibiting the formation of mold deposits during molding as completely as possible, though the heat stability was improved by the addition.
Polyoxymethylene generally has an essential property of being easily decomposed in a hot oxidizing atmosphere or under an acidic or alkaline condition owing to its chemical structure, so that the addition of an antioxidant and other stabilizer thereto is indispensable in practice. However, a polyoxymethylene composition containing these stabilizers is not sufficiently prevented from suffering decomposition. Therefore, when the molding work is prolonged, powdery or tarry substances (mold deposits) adhere to the mold surface, which is one of the chief factors lowering the working efficiency and deteriorating the surface appearance of a molded article.